03 mercury optimax 150 overheat questions

Hello, on the main boat I use all the time I am having overheat issues. If you start out and run at normal planning speed all seems fine. When you let off the throttle and idle for a minute or so and go to pick up speed again, around 1krmp to 2krpm the gaurdian system kicks in and limits power and the alarm sounds. Idling back down makes the beep go away. Have to idle back to the dock as if I try to throttle up the same problem repeats itself. Impeller is good, stream out the back is good, increases with rpm. Thermostats appear to be ok, as removing hoses the water begins to pump about 140 to 150 area. water pressure at thermostats increase with rpm. center line going to poppet has a steady flow of water. Once concern is that I can blow through that line to the poppet with no resistance, so I think that has failed open. I dont know if this would cause too much water passing through there and not cooling the heads properly? Any ideas on what the problem may be? Thanks